06 additional hec-ras modeling

Additional HEC-RAS Modeling Features

RAS Optional Capabilities

ObjectiveTo allow you to become familiar with some of the optional capabilities in HEC-RAS


Optional CapabilitiesMultiple Plan AnalysisEffective Area OptionCross Section InterpolationGraphical Editing FeaturesMixed Flow Regime CalculationsFlow Distribution CalculationsInline Weirs and Gated Spillways


Multiple Plan Analysis


Notes on PlansModifications can be made to the geometry and/or flow data, and then saved in separate files. Plans are formulated by selecting a particular geometry file and a particular flow file. The multiple plan option is useful when, for example, a comparison of existing conditions and future channel modifications are to be analyzed.


Multiple Plan OptionCan be used to perform a design of a specific geometric feature. For example, if you are sizing a bridge opening, a separate geometry file could be developed for a base condition (no bridge), and then separate geometry files could be developed for each possible bridge configuration. A plan would consist of a flow file and one of the geometry files. Computations are performed for each plan individually.


Viewing tables and graphsOnce the computations are performed for all the plans, the user can then view output in a graphical and tabular mode for any single plan or combination of plans.


Ineffective flow areasKE = aV2/2g, where V =Q/A

Velocity head is associated with flow in downstream direction If area in the cross section is not carrying flow it should not be includedThe ineffective area option of RAS is used for this


Example:Ineffective Flow Areas at a BridgeFour cross sections are needed to describe the active flow area for flow through the bridge opening


Designation of Ineffective Area Related to Type of Bridge FlowTypes of Bridge FlowLow flow through bridge openingPressure flow through bridge openingWeir flow Pressure and weir flowHigh flows over bridge and approaches


Cross Sections at BridgeLocation of Cross Sections 2 and 3Bounding sections for bridgeDefine bridge interior for upstream and downstream bridge sectionsMust be defined before bridge data are inputUsually have ineffective flow area defined


Ineffective Flow Areas at Cross Section 2


Defining Ineffective AreaPermanently block out areaBlock out area until water surface reaches a given elevationMultiple blocksUse a leveeUse very high n-value in regions with very low velocity


Normal Blocked ObstructionsBlocked obstructions decrease flow area and add wetted perimeter when the water comes in contact with the obstruction


Multiple Blocked ObstructionsUp to 20 individual blocks can be defined. The left station, right station, and an elevation are entered for each of the blocks.


Ineffective Area OptionThe area is included in the storage calculations, but it is not included as part of the active flow area. No additional wetted perimeter is added to the active flow area.


Levee OptionNo water can go to the left of the left levee station or to the right of the right levee station until the levee elevations are exceeded.


Levee AddedLeft Levee Station and ElevationA vertical wall is placed up to the established levee height. Additional wetted perimeter is included when water comes into contact with the levee wall.


Cross Section InterpolationMay Be Needed When: The change in velocity head is too largeTo better model friction lossesTo prevent the program from defaulting to critical depthTo generate a smoother plot of the water surface profile


Cross Section Interpolation


Interpolation within a Reach


Interpolation between 2 Sections


Adding Additional Master Cords


Cross Section InterpolationCAUTION: Automatic interpolation should not be used as a replacement for required cross section data!Interpolation can improve model calculations (especially for unsteady flow analysis)


Graphical Editing Features


Modified geometry


Mixed Flow CalculationsSteady Flow CalculationsSubcritical FlowSupercritical FlowMixed FlowUnsteady Flow CalculationsSubcritical FlowSupercritical FlowMixed Flow


Mixed Flow


Specific Force Equation


Flow Distribution Option The flow distribution option allows the user to see in different parts of the cross section:The amount of flow The flow velocity


Setting the Flow Distribution Locations


Plotting Velocity Distribution


Velocity Distribution Plot


Flow Distribution Output


CAUTIONFlow distribution varies vertically and horizontally (all flows are 3-D in nature )

HEC-RAS is a 1-D model

Flow distribution is based on area and wetted perimeter of each flow slice (using fewer slices is generally better)


Inline Weirs and Gated Spillways


Sluice and Radial GatesGate openings can handle both submerged and unsubmerged conditions


Cross Section Layout


Inline Weir Editor


Weir/Embankment


Gate Editor


Setting the Gate Opening


RecapWe covered:Multiple Plan AnalysisEffective Area OptionCross Section InterpolationFlow RegimesFlow Distribution CalculationsInline Weirs and Gated Spillways


RAS FeaturesRAS FeaturesRAS FeaturesThe HEC-RAS system has the ability to compute a series of water surface profiles for a number of different characterizations (Plans) of the river system. Modifications can be made to the geometry and/or flow data, and then saved in separate files. Plans are then formulated by selecting a particular geometry file and a particular flow file. The multiple plan option is useful when, for example, a comparison of existing conditions and future channel modifications are to be analyzed. Channel modifications can consist of any change in the geometric data, such as: the addition of a bridge or culvert; channel improvements; the addition of levees; changes in n values due to development or changes in vegetation; etc... The multiple plan option can also be used to perform a design of a specific geometric feature. For example, if you were sizing a bridge opening, a separate geometry file could be developed for a base condition (no bridge), and then separate geometry files could be developed for each possible bridge configuration. A plan would then consist of selecting a flow file and one of the geometry files. Computations are performed for each plan individually. Once the computations are performed for all the plans, the user can then view output in a graphical and tabular mode for any single plan or combination of plans.

RAS FeaturesRAS FeaturesRAS FeaturesReferring to the Figure above, the dashed lines represent the effective flow boundary for low flow and pressure flow conditions. For cross sections 2 and 3, ineffective flow areas to either side of the bridge opening should not be included as part of the active flow area for low flow or pressure flow.

The ineffective area option is used at sections 2 and 3 to keep all the flow in the area of the bridge opening while all the flow is passing under the bridge. The elevations specified for ineffective flow should correspond to elevations where significant weir flow passes over the bridge.The ineffective area option allows the overbank areas to become effective as soon as the ineffective-area elevations are exceeded. The flow in the overbank will pass over the bridge-roadway. RAS FeaturesRAS FeaturesNote, the cross section River Station is merely an identifier. Distances between sections and to the bridge deck are specified in the bridge editor.RAS FeaturesCross Section 2. Cross section 2 is located a short distance downstream from the structure. The computed water surface at this cross section will represent the tailwater elevation of the weir and the gated spillways. This cross section should not include any of the structure or embankment, but represents the physical shape of the channel just downstream of the structure. The shape and location of this cross section is entered separately from the Inline Weir and Gated Spillway data (from the cross section editor).

The HECRAS ineffective area option is used to restrict the effective flow area of cross section 2 to the flow area around or near the edges of the gated spillways, until flow overtops the overflow weir and/or embankment. The ineffective flow areas are used to represent the correct amount of active flow area just downstream of the structure. Establishing the correct amount of effective flow area is very important in computing an accurate tailwater elevation at cross section 2. Because the flow will begin to expand as it exits the gated spillways, the active flow area at section 2 is generally wider than the width of the gate openings. The width of the active flow area will depend upon how far downstream cross section 2 is from the structure. In general, a reasonable assumption would be to assume a 1:1 expansion rate over this short distance.

Cross sections 1 and 2 are located so as to create a channel reach downstream of the structure in which the HECRAS program can accurately compute the friction losses and expansion losses that occur as the flow fully expands.RAS FeaturesRAS FeaturesThis option defines areas of the cross section that will be permanently blocked out. Blocked obstructions decrease flow area and add wetted perimeter when the water comes in contact with the obstruction. A blocked obstruction does not prevent water from going outside of the obstruction.

Two alternatives are available for entering blocked obstructions:

1. Normal blocked areas: When this option is used, the area to the left of the left station and to the right of the right station will be completely blocked out. RAS Features2. Multiple Blocked Obstructions: Up to 20 individual blocks can be defined. The left station, right station, and an elevation are entered for each of the blocks. RAS FeaturesIneffective Flow Areas - Defines areas of the cross section that will contain water that is not actively being conveyed (ineffective flow).

Used to describe portions of a cross section in which water will pond, but the velocity, in the downstream direction, is close to zero. The area is included in the storage calculations and other wetted cross section parameters, but it is not included as part of the active flow area and no additional wetted perimeter is added to the active flow area.

This example shown above is a Normal Ineffective Area, a second example is the Blocked Ineffective Area.

RAS FeaturesRAS provides the ability to establish a left and/or right levee station and elevation on any cross section. When established, no water can go to the left of the left levee station or to the right of the right levee station until either of the levee elevations are exceeded.

Levee stations must be defined explicitly, or the program assumes that water can go anywhere within the cross section. An example of a cross section with a levee on the left side is shown in the figure.RAS FeaturesA levee can be added into a data set in order to see what effect a levee will have on the water surface by setting a levee station and elevation that is above the existing ground. A vertical wall is placed at that station up to the established levee height. Additional wetted perimeter is included when water comes into contact with the levee wall. RAS FeaturesOccasionally it is necessary to supplement surveyed cross section data by interpolating cross sections in between two surveyed sections. Interpolated cross sections are often required when the change in velocity head is too large to accurately determine the change in the energy gradient. An adequate depiction of the change in energy gradient is necessary to accurately model friction losses as well as contraction and expansion losses. When cross sections are spaced too far apart, the program may end up defaulting to critical depth.

The HEC-RAS program has the ability to generate cross sections by interpolating the geometry between two user entered cross sections. The geometric interpolation routines in HEC-RAS are based on a string model, as shown on the next slide.RAS FeaturesThe string model in HEC-RAS consists of cords that connect the coordinates of the upstream and downstream cross sections. The cords are classified as "Master Cords" and "Minor Cords". Master cords are defined explicitly as to number and starting and ending location of each cord. The five default master cords are based on the following location criteria:1. First coordinate of the cross section (May be equal to left bank).2. Left bank of main channel (Required to be a master cord).3. Minimum elevation point in the main channel.4. Right bank of main channel (Required to be a master cord).5. Last coordinate of the cross section (May be equal to right bank).The interpolation routines are not restricted to a set number of master cords. At a minimum there must be two master cords, but there is no maximum. Additional master cords can be added by the user. The minor cords are generated automatically by the interpolation routines. A minor cord is generated by taking an existing coordinate in either the upstream or downstream section and establishing a corresponding coordinate at the opposite cross section by either matching an existing coordinate or interpolating one. The station value at the opposite cross section is determined by computing the proportional distance that the known coordinate represents between master chords, and then applying the proportion to the distance between master cords of the opposite section. The number of minor cords will be equal to the sum of all the coordinates in the upstream and downstream sections minus the number of master cords.Once all the minor cords are computed, the routines can then interpolate any number of sections between the two known cross sections. Interpolation is accomplished by linearly interpolating between the elevations at the ends of a cord. Interpolated points are generated at all of the minor and master cords. The elevation of a particular point is computed by distance weighting, which is based on how far the interpolated cross section is from the user known cross sections.

RAS FeaturesThe first cross section interpolation option, Within a Reach, allows for automatic interpolation over a specified range of cross sections within a single reach. When this option is selected, a window will pop up as shown above. The user must first select the River and Reach that they would like to perform the interpolation in. Next the user must enter a starting River Station and an ending River Station for which interpolation will be performed. The user must also provide the maximum allowable distance between cross sections. If the main channel distance between two sections is greater than the user defined maximum allowable, then the program will interpolate cross sections between these two sections. The program will interpolate as many cross sections as necessary in order to get the distance between cross sections below the maximum allowable.

Once the user has selected the cross section range and entered the maximum allowable distance, cross section interpolation is performed by pressing the Interpolate XS's button. When the program has finished interpolating the cross sections, the user can close the window by pressing the Close button. Once this window is closed, the interpolated cross sections will show up on the river schematic as light green tic marks. The lighter color is used to distinguish interpolated cross sections from user entered data. Interpolated cross sections can be plotted and edited like any other cross section. The only difference between interpolated sections and user defined sections is that interpolated sections will have an asterisk (*) attached to the end of their river station identifier. This asterisk will show up on all input and output forms, enabling the user to easily recognize which cross sections are interpolated and which are user defined.

RAS FeaturesThe same interpolation scheme is used in both of the automated interpolation options ("Within a Reach" and "Between 2 XS's"). The difference is that the Between 2 XS's option allows the user to define additional master cords. This can provide for a better interpolation, especially when the default of five major cords produces an inadequate interpolation. An example of an inadequate interpolation when using the default cords is shown above.

As can be seen, the interpolation was adequate for the main channel and the left overbank area. The interpolation in the right overbank area failed to connect two geometric features that could be representing a levee or some other type of high ground. If it is known that these two areas of high ground should be connected, then the interpolation between these two sections should be deleted, and additional master cords can be added to connect the two features. To delete the interpolated sections, press the Del Interp button.RAS FeaturesMaster cords are added by pressing the Master Cord button that is located to the right of the Maximum Distance field above the graphic. Once this button is pressed, any number of master cords can be drawn in. Master cords are drawn by placing the mouse pointer over the desired location on the top cross section. Then while holding the left mouse button down, drag the mouse pointer to the desired location of the lower cross section. When the left mouse button is released, a cord is automatically attached to the closest point near the pointer.User defined master cords can also be deleted. To delete user defined master cords, press the scissors button to the right of the master cords button. When this button is pressed, simply move the mouse pointer over a user defined cord and click the left mouse button to delete the cord.Once you have drawn in all the master cords that you feel are required, and entered the maximum distance desired between sections, press the interpolate button. When the interpolation has finished, the interpolated cross sections will automatically be drawn onto the graphic for visual inspection.In general, the best approach for cross section interpolation is to first interpolate sections using the "Within a Reach" method. Next, all of the interpolated sections should be viewed to ensure that a reasonable interpolation was accomplished in between each of the cross sections. This can be done from the "Between 2 XS's" window. Whenever the user finds interpolated cross sections that are not adequate, they should be deleted. A new set of interpolated sections can then be developed by adding additional master cords in order to improve the interpolation.RAS FeaturesCAUTION: Automatic geometric cross section interpolation should not be used as a replacement for required cross section data. If water surface profile information is required at a specific location, surveyed cross section data should be provided at that location. It is very easy to use the automatic cross section interpolation to generate cross sections. But if these cross sections are not an adequate depiction of the actual geometry, you may be introducing error into the calculation of the water surface profile. Whenever possible, use topographic maps to assist you in evaluating whether or not the interpolated cross sections are adequate. Also, once the cross sections are interpolated, they can be modified just like any other cross section.

If the geometry between two surveyed cross sections does not change linearly, then the interpolated cross sections will not adequately depict what is in the field. When this occurs, the modeler should either get additional surveyed cross sections, or adjust the interpolated sections to better depict the information from the topographic map.RAS FeaturesRAS FeaturesThe mixed flow regime calculations in HEC-RAS are performed as follows:

First, a subcritical water surface profile is computed starting from a known downstream boundary condition.

Next, the program attempts to compute a supercritical profile. The program will start with the upstream end and compute a supercritical profile as long as the supercritical profile has a greater specific force than the previously computed subcritical answer at that location.

If, at a given location, the subcritical water surface has a greater specific force, then the program will keep the subcritical answer. In this case, the program will start searching downstream for a cross section that has a critical depth answer. From the critical depth cross section, the program will once again compute a supercritical profile as long as the supercritical answer has a greater specific force.RAS FeaturesQ= Discharge at each sectionB= Momentum coefficient (similar to alpha)A= Total flow areaY= Depth from the water surface to centroid of the areag= Gravitational accelerationRAS FeaturesRAS FeaturesThe flow distribution output can be obtained by first defining the locations that the user would like to have this type of output. As shown above, the user can either select specific locations or all locations in the model. Next, the number of slices for the flow distribution computations must be defined for the left overbank, main channel, and the right overbank. The user can define up to 45 total slices. Each flow element (left overbank, main channel, and right overbank) must have at least one slice. The user can change the number of slices used at each of the cross sections. The flow distribution output will be calculated for all profiles in the plan during the computations.

The final step is to perform the normal profile calculations. During the computations, at each cross section where flow distribution is requested, the program will calculate the percentage of flow, area, wetted perimeter, conveyance, and average velocity for each of the user defined slices.RAS FeaturesThe user has the option of plotting velocity distribution output from the cross section viewer. Velocity distributions can only be plotted at locations in which the user specified that flow distribution output be calculated during the computations. To view the velocity distribution plot, first bring up a cross section plot (select Cross Sections from the view menu of the main HEC-RAS window). Next, select the cross section in which you would like to see the velocity distribution output. Select Velocity Distribution form the Options menu of the cross section window. This will bring up a popup window that will allow you to set the minimum velocity, maximum; velocity, and velocity increment for plotting. In general, it is better to let the program use the maximum velocity range for plotting. Next, the user selects Plot Velocity Distribution. Then press the OK button and the velocity distribution plot will appear.RAS FeaturesRAS FeaturesThe Flow Distribution table type can be used to view the computed flow distribution output at any cross section where this type of output was requested. To bring up the flow distribution table, first select Cross Section Table from the View menu of the main HEC-RAS window. Once the cross section table is on the screen, the user can select Flow Distribution table from the Type menu of the cross section table.RAS FeaturesIn general, the results of the flow distribution computations should be used cautiously. Specifically, the velocities and discharges are based on a one-dimensional hydraulic model. A true velocity and flow distribution varies vertically as well as horizontally. To achieve such detail, the user would need to use a three-dimensional hydraulic model, or go out and measure the flow field. While the results for the flow distribution are better than the standard three subdivisions (left overbank, main channel, and right overbank) provided by the model, the values are still based on average estimates of the one-dimensional results. Also, the results obtained from the flow distribution option can vary with the number of slices used for the computations. In general, it is better to use as few slices as possible.RAS FeaturesHEC-RAS allows the user to model inline gated spillways and overflow weirs. HEC-RAS has the ability to model radial gates (often called tainter gates) or vertical lift gates (sluice gates). The spillway crest of the gates can be modeled as either an ogee shape or a broad crested weir shape. In addition to the gate openings, the user can also define a separate uncontrolled overflow weir.

The overflow weir is entered as a series of station and elevation points across the stream, which allows for complicated weir shapes. The user must specify if the weir is broad crested or an ogee shape. The software has the ability to account for submergence due to the downstream tailwater. Additionally, if the weir has an ogee shaped crest, the program can calculate the appropriate weir coefficient for a given design head. The weir coefficient will automatically be decreased or increased when the actual head is lower or higher than the design head.RAS FeaturesGated Spillways within HEC-RAS can be modeled as radial gates (often called tainter gates) or vertical lift gates (sluice gates). The equations used to model the gate openings can handle both submerged and unsubmerged conditions at the inlet and outlet of the gates. If the gates are opened far enough, such that unsubmerged conditions exist at the upstream end, the program automatically switches to a weir flow equation to calculate the hydraulics of the flow. The spillway crest through the gate openings can be specified as either an ogee crest shape or a broad crested weir. The program has the ability to calculate both free flowing and submerged weir flow through the gate openings.

Up to 10 gate groups can be entered into the program at any one river crossing. Each gate group can have up to 25 identical gate openings. Identical gate openings must be the same gate type; size; elevation; and have identical gate coefficients. If anything about the gates is different, except their physical location across the stream, the gates must be entered as separate gate groups.RAS FeaturesThe inline weir and gated spillway routines in HEC-RAS require the same cross sections as the bridge and culvert routines. Four cross sections in the vicinity of the hydraulic structure are required for a complete model, two upstream and two downstream. In general, there should always be additional cross sections downstream from any structure (bridge, culvert, weir, etc...), such that the user entered downstream boundary condition does not affect the hydraulics of flow through the structure. In order to simplify the discussion of cross sections around the inline weir and gated spillway structure, only the four cross sections in the vicinity will be discussed. These four cross sections include: one cross section sufficiently downstream such that the flow is fully expanded; one at the downstream end of the structure (representing the tail water location); one at the upstream end of the structure (representing the headwater location); and one cross section located far enough upstream at the point in which the flow begins to contract. Note, the cross sections that bound the structure represent the channel geometry outside of the embankment.Cross Section 1. Cross section 1 for a weir and/or gated spillway should be located at a point where flow has fully expanded from its constricted top width caused by the constriction. The entire area of cross section 1 is usually considered to be effective in conveying flow.RAS FeaturesInline weir and gated spillway data are entered in a similar manner as bridge and culvert data. To enter an inline weir and/or gated spillway press the Inline Weir/Spillway button from the Geometric Data window. Once this button is pressed, the Inline Weir and Gated Spillway Data editor will appear as shown above (except yours will be blank until you have entered some data).To add an inline weir and/or gated spillway to a model, the user must do the following:Select the river and reach that you would like to place this inline weir and/or spillway into. This is accomplished by first selecting a River, then selecting a specific reach within that river. The River and Reach selection buttons are at the top of the Inline Weir and/or Gated Spillway Data editor.Go to the Options menu at the top of the window and select Add an Inline Weir and/or Gated Spillway from the list. An input box will appear asking you to enter a river station identifier for locating this structure within the reach. After entering the river station, press the OK button and a copy of the cross section just upstream of this river station will appear on the screen. This cross section is used in formulating the inline weir and/or gated spillway crossing.Enter all of the data for the Inline Weir and/or Gated Spillway. This data will include a Weir/Embankment profile, and any gated spillways that you may be modeling. Gated spillways are optional.RAS FeaturesThe weir information is entered in a similar manner to the bridge and culvert data.

The station data is entered from left to right. Everything below this elevations will be filled in down to the ground.

Min Weir Flow El - This field is used to set the minimum elevation for which weir flow will begin to be evaluated. Once the computed upstream energy becomes higher than this elevation, the program begins to calculate weir flow. However, the weir flow calculations are still based on the actual geometry of the weir/embankment, and are not effected by this elevation. If this field is left blank, the elevation that triggers weir flow is based on the lowest elevation of the station and elevation coordinates.

RAS FeaturesGate Group - The Gate Group is automatically assigned to "Gate #1" the first time you open the editor. The user can enter up to 10 different Gate Groups at each particular river crossing, and each gate group can have up to 25 identical gate openings. If all of the gate openings are exactly the same, then only one gate group needs to be entered. If the user has gate openings that are different in shape, size, elevation, or have different coefficients, then additional Gate Groups must be added for each Gate type. Also, if the gates are identical, but the user wants to be able to open the gates to different elevations, then the user must have a separate gate group for each set of gates that will be opened to different elevations.

Centerline Stations - The user should enter a different centerline stationing for each gate opening that is part of the current gate group. All gate openings within the same gate group are exactly identical in every way, except their centerline stationing. As a user adds new centerline stationing values, the number of identical gates in the group is automatically incremented and displayed in the field labeled # Openings.

Weir Data and Coefficient When the gate is opened to an elevation higher than the upstream water surface, the flow is treated as weir flow and the program uses the Weir Data information. For sluice type flow, the program uses the Discharge Coefficient. When the flow is fully submerged, the Orifice Coefficient is used.RAS FeaturesThe number of gates that are open, and the opening height, is set for each gate group for each profile.RAS FeaturesRAS Features

06 additional hec-ras modeling

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